1. Field of the Invention
The present invention relates to a multilayer optical recording medium with a plurality of stacked recording and reading layers from which information can be read by light irradiation.
2. Description of the Related Art
In the field of optical recording media, recording density has been increased by shortening the wavelengths of laser light sources or by increasing the numerical apertures of optical systems. With reference, for example, to optical recording media in accordance with the standards for Blu-ray Disc (BD), recording and reading of the capacity of 25 GB to and from one layer has been allowed by setting the wavelength of laser at 405 nm and the numerical aperture at 0.85. However, efforts by light sources and optical systems have reached their limits. A volumetric recording system in which information is multiply recorded in the direction of an optical axis has been desired in order to increase recording capacity further. By way of example, optical recording media each with eight recording and reading layers (see Non-Patent Literature 1), or with six recording and reading layers (see Non-Patent Literature 2) have been suggested as those in accordance with the standards for Blu-ray Disc (BD).
Multilayer optical recording media may suffer from mixing a signal of other recording and reading layers into a target recording and reading layer, or leakage of noise generated by the effect of other recording and reading layers into a target recording and reading layer, during the reading of information from the target recording and reading layer. Such mixing or leakage problems generally referred to as crosstalk result in degradation of a servo signal or a recording signal.
The crosstalk includes two types including interlayer crosstalk and confocal crosstalk. The interlayer crosstalk is a phenomenon produced by mixing of light reflected off a recording and reading layer next to a recording and reading layer being read with reading light. Accordingly, the interlayer crosstalk is always a matter of concern in multilayer optical recording media with two or more recording and reading layers. The interlayer crosstalk is reduced by increasing an interlayer distance.
The confocal crosstalk is specific to multilayer optical recording media with three or more recording and reading layers. The confocal crosstalk is a phenomenon produced by coincidence in optical path length between primary reading light reflected off a recording and reading layer being read only once, and stray light reflected off a different recording and reading layer a plurality of times.
Principles of generation of the confocal crosstalk are described with reference to FIGS. 11 to 14. In a multilayer optical recording medium 40 shown in FIG. 11, a beam 70 focused on an L0 recording and reading layer 40d for reading or recording is split into a plurality of optical beams due to semi-light-transmitting properties of recording and reading layers. FIG. 12 shows a phenomenon where a beam 71 branching off from a beam targeted for recording and reading to and from an L0 recording and reading layer 40d is reflected off an L1 recording and reading layer 40c and is focused on an L2 recording and reading layer 40b, and the resulting reflected light is detected after being reflected off the L1 recording and reading layer 40c again.
FIG. 13 shows a phenomenon where a beam 72 branching off from a beam targeted for recording and reading to and from an L0 recording and reading layer 40d is reflected off an L2 recording and reading layer 40b and is focused on a light incident surface 40z, and the resulting reflected light is detected after being reflected off the L2 recording and reading layer 40b again. FIG. 14 shows a phenomenon where a beam 73 branching off a beam targeted for recording and reading to and from an L0 recording and reading layer 40d is not focused on a different recording and reading layer, but is detected after being reflected off L1, L3 and L2 recording and reading layers 40c, 40a and 40b in this order.
The light intensity of the beams 71 to 73 as stray light are smaller than that of the beam 70. However, the beams 71 to 73 enter a photodetector with the same optical path length and with the same radius of light flux, generating influential interference. Accordingly, the amount of light received by the photodetector can vary largely in response to the minute change of an interlayer thickness, making it difficult to detect a stable signal. Meanwhile, the amount of stray light determined by the product of the respective reflectances of recording and reading layers decreases as the stray light is reflected a greater number of times. Accordingly, for practical purposes, considering stray light reflected off multiple surfaces three times is sufficient.
In the phenomena shown in FIGS. 11 to 14, the beams 70 and 71 have the same optical path length and the same radius of light flux if T1 is set to be equal to T2. In this case, the beams 70 and 71 enter the photodetector and are detected at the same time. Likewise, the beams 70 and 72 have the same optical path length and the same radius of light flux if the total of T1 and T2 is set to be equal to the total of T3 and TC. Also, the beams 70 and 73 have the same optical path length and the same radius of light flux if T3 is set to be equal to T1. Accordingly, making all interlayer distances different is a generally employed technique to avoid the confocal crosstalk.
Non-Patent Literature 1: Ichimura et. al., Appl. Opt, 45, 1974-1803 (2006), and Non-Patent Literature 2: K. Mishima et al., Proc. of SPIE, 6282, 628201 (2006) are introduced as the Prior Art Document.
As described above, the interlayer crosstalk may be avoided by increasing an interlayer distance. This however makes it difficult to increase the number of stacked recording and reading layers in the range of a limited thickness. Also, the confocal crosstalk may be avoided by making all interlayer distances different while the number of stacked recording and reading layers is increased. This however requires intermediate layers with various film thicknesses, leading to greater interlayer distances. As a result, a distance of a recording and reading layer, which is farthest from a light incident surface, from the light incident surface is increased, thereby causing an adverse effect on a comma aberration due to a tilt and the like.
Furthermore, in some cases, concavities and convexities for tracking control such as grooves and lands should be formed in each recording and reading layer. In these cases, concavities and convexities should be formed in each intermediate layer with a stamper, so that an error is likely to be generated in the film thicknesses of the intermediate layers. The respective intermediate layers may be set to have different film thicknesses in consideration of the effect of such an error generated during film deposition in advance. This however requires setting of a rather large difference between film thicknesses, resulting in more and more greater thickness of a multilayer optical recording medium.
In order to facilitate control by a recording and reading unit, recording and reading layers in a multilayer optical recording medium are generally configured to have the same reflectance determined in a stacked state of the recording and reading layers (that is a reflectance determined by a ratio between incident light and reflected light when each recording and reading layer in the completed multilayer optical recording medium is irradiated with light). Or alternatively, the recording and reading layers are generally configured to be irradiated with laser light having approximate values of laser power applied during recording. In order to achieve these, each of the recording and reading layers should be made of an optimized material, and should have an optimized film structure, an optimized film thickness and the like. This requires the recording and reading unit to set an optimum recording condition (such as recording strategy and the waveform of an irradiation pulse, for example) for each layer. In any case, the conventional idea generates the fear of increased burdens on both a side to manufacture a multilayer optical recording medium and a side to design a recording and reading unit in response to the increase of the number of stacked recording and reading layers.